Injured No. 1 Serena Williams follows Azarenka in quitting Dubai

The 31-year-old Williams, who became the oldest women's tennis player to be crowned world No. 1 on Monday, said she was forced out because of a back injury.

"I've just had some back problems the past couple of weeks," the American told an impromptu press conference.

"I thought it would get better as the week went on but it didn't. I don't want to keep pushing it and make it worse."

Williams took to the court to apologize to fans who had arrived for her second-round match with Marion Bartoli of France.

The absences of both Williams and previous No. 1 Azarenka represent a severe dent to a popular $2 million tournament which had already been without the sport's two reported highest earners, Maria Sharapova and Li Na (according to Forbes magazine).

It is also a blow to the Women's Tennis Association, which has worked hard to introduce incentives and regulations to reduce the number of withdrawals from its events.

Wednesday's unexpected withdrawal follows that of second-ranked Azarenka, which ensured that Williams will hang on to top spot until the Sony Ericsson Open, which starts in Miami on March 18.

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